I have a t-mobile phone and I just bought a cingular blackberry from ebay do...

No your t-mobile sim wont work in Blackberry phone until you unlock it from Cingular network provider..You can easily unlock the phone using sites like SimpleUnlocking.com here they are rendering network unlock code for all mobile models at reliable cost .You first check that whether the code is available for your mobile model ,country and network provider .If yes then pay for it and unlock your phone then freely use it with any GSM sim ..